The x values are the same in all 4 answer choices.  We need to focus on the y-values!
One of the best definitions of a linear function is “every time x changes by 1, y changes by the same amount”.  We can determine this by looking at the differences in the y values or f(x), g(x), h(x) or k(x)
(1) f(x): each time the x value goes up by 1 the y value goes up by how much?
(2) g(x) is each change the same?  (from -1 to 1 is a difference of 2 but from 1 to 5 is a difference of 4)
(3) h(x) is each change the same?  from 9 to 12, it changes by 3 but then from 12 to 17, it changes by 5
(4) k(x), is each change the same?  from -2 to 4, the change is 6, but from 4 to 14 the change is 10

That’s it — you just found the linear function!! 2 points!